Field of the Invention
This invention relates to a blade for a turbomachine impeller comprising an airfoil formed with a pressure surface, a suction surface, a trailing edge, and a leading edge, and a platform extending at one of the ends of the airfoil in a direction which is globally perpendicular to a longitudinal direction of the airfoil, the blade being adapted to be arranged with a plurality of substantially identical blades to form a ring around a ring axis and define therealong an upstream and a downstream area, wherein the airfoils are arranged substantially radially in the ring, and the adjacent blade platforms join in pairs so as to form an inter-airfoil surface linking the pressure surface of an airfoil to the suction surface of the neighboring airfoil.
Description of the Related Art
Joining such blades around a common axis allows for composing an impeller, the axis of which is the axis of the ring. This impeller may be mobile, and thus receive energy from the jet, or communicate energy to the jet traveling through the impeller; it may also be fixed, and in this case, its function is to canalize the jet.
The blade can be a distinct part as such, or integrated with other blades so as to form for instance a distributor sector or a multiple bladed disk.
Usually, a turbomachine comprises several blade stages, forming a series of fixed or mobile impellers, successively arranged along the fluid path through the turbomachine (There may be several paths, especially in the case of bypass engines). The efficiency of the turbomachine is directly related to the capability of each of the impellers, and thus in particular each of the blades belonging thereto, to efficiently interact with the jet, i.e. without unnecessarily dissipating energy. It should be noted that namely in aeronautical turbomachines, such as turbojets or turboprops, jet speeds may be significant, namely supersonic: for a blade arranged in such a jet, it is essential to optimize the flow quality of the jet around the blade.
In the blade, the shape of the airfoil must naturally be optimized so as to efficiently guide the jet, in which the airfoil is located, or to receive or transmit maximum energy to the jet without dissipating energy by heating.
However, although the shape of the airfoil is important, it has been found that the shape of the surface of the platform on the side of the airfoil also plays an essential part for the flow quality of the jet through the blade. Thus, the phenomena, which the platforms of an impeller may affect can account for 30% of the total losses thereat.
For the sake of simplicity, in the following, a platform surface designates the surface of the platform on the side of the airfoil, without repeating on which side of this surface it is located.
The passage of the jet around the blades as those indicated in the preamble is illustrated in FIGS. 1 and 2.
FIG. 1 shows three identical blades 10, which are part of an impeller 100 presented in FIG. 2. Each blade 10 is designed to be assembled with other identical blades 10 so as to form an impeller 100. This impeller is essentially composed of the blades 10 mounted on a rotor disk 20. In this impeller 100, the blades 10 are mounted periodically around the axis A of the wheel. Globally, the fluid jet flows along the axis A of an upstream side to a downstream side of the impeller.
Each blade 10 comprises an airfoil 50, a platform 60, as well as a root 66 in the represented specific case of a rotor blade for fixing the blade to a rotor disk. The platform 60 extends in a direction which is globally perpendicular to the longitudinal direction of the airfoil 50 and comprises a platform surface 62 on the side of the airfoil. As the blades 10 are assembled against each other, the platforms thereof join in pairs so as to create a substantially continuous surface, the so-called ‘inter-airfoil’ surface 70 extending from the pressure surface 56 of one airfoil to the suction surface 58 of the neighboring airfoil. Thus, the inter-airfoil surface groups the adjacent portions of the platform surfaces 62 of two adjacent blades 10, 10′ located between their respective airfoils 50. The platform surface 62 is linked to the outer surfaces of the airfoil 50 by connecting surfaces 18 (which are substantially connecting fillets having a tapered radius).
It should also be noted that in the examples represented in FIGS. 1 to 3, the surface 62 of the platform 60 is a surface of revolution, i.e. that the area thereof is substantially part of a surface of revolution around the axis A of the impeller. Herein, a surface of revolution around an axis designates a surface generated by rotating a curve around said axis. Such a shape is common for blade platform surfaces for turbomachine impellers.
In the flow, when the jet reaches the leading edge of an airfoil 50, it splits in two, going partly past the side of the pressure surface 56 and partly past the side of the suction surface 58 of the airfoil 50. FIG. 3 schematically presents how the pressure field is established in the ‘inter-airfoil channel’ 30 extending between the airfoils.
FIG. 3 is a sectional view perpendicular to the respective axes of the airfoils of two blades 10 and 10′ mounted side by side in an impeller. More particularly, FIG. 3 shows approximately the pressure field which can usually be observed close to the inter-airfoil surface 70 between the suction surface 58 of a first airfoil and the pressure surface 56′ of a second airfoil.
FIG. 3 comprises an iso-pressure curve 40 corresponding to a relatively high pressure, and an iso-pressure curve 42 corresponding to a relatively low pressure, these pressures being observed in the jet during operation of the turbomachine. A steep pressure gradient J is created between the pressure surface and the suction surface of the two airfoils due to pressure being greater close to the pressure surface than close to the suction surface. Under the effect of this pressure gradient J, a transverse flow to the ‘inter-airfoil’ channel 30 is generated at the root (and head) of the airfoils, and particles thus deflected are pushed towards the suction surface of the airfoil 50. Thereby, within the ‘inter-airfoil’ channel 30, strong secondary flows not directed in the main direction of flow are created which will generate eddies, namely close to the suction surface.
In order to try to limit unnecessary dissipation of energy resulting therefrom close to the inter-airfoil surface, the U.S. Pat. No. 7,220,100 proposes an inter-airfoil surface shape comprising mainly a convex ramp located immediately adjacent to the pressure surface of the airfoil, and a concave area located immediately adjacent to the suction surface of the airfoil, each of these areas being located substantially at the mid-point of the airfoil chord. In spite of this development, there is still a number of energy dissipating eddies in the space between the two airfoils, and therefore, there is a need for a blade shape further reducing the stray eddies formed in this space.
The U.S. Pat. No. 6,283,713 proposes another shape for the inter-airfoil surface, on the one hand comprising a convex region adjacent to the suction surface of the blade, and a concave region adjacent to the pressure surface of the blade, with these two regions having a significant dimension as they extend over most of the length of the chord of the blade. According to an alternative, the blade comprises at the trailing edge a boss and a recess, respectively located on the side of the suction surface and the pressure surface. However, these configurations of the inter-airfoil surface do not allow for the problem of unnecessary energy dissipation near this surface to be solved efficiently.